1. Field of the Invention
The present invention relates to an optical system including a diffractive optical element (diffractive grating), and particularly relates to an optical system suitable for optical apparatuses such as image pickup apparatuses.
2. Description of the Related Art
As a method for reducing chromatic aberration generated in optical systems, such as chromatic aberration of magnification and longitudinal chromatic aberration, Japanese Patent Laid-Open Nos. 2006-003913 and 11-271514 disclose an achromatizing method using a diffractive optical element (DOE). Optical systems disclosed in Japanese Patent Laid-Open Nos. 2006-003913 and 11-271514 achieve achromatization by using (a) an aspheric surface effect obtained by varying a periodic structure of the DOE and (b) a chromatic aberration reduction effect obtained by utilizing a negative dispersion characteristic (νd=−3.453) and a strong anomalous dispersion characteristic (θgF=0.296) of the DOE, which are different from those of general glass materials. Such an achromatizing method enables not only the reduction of the chromatic aberration but also miniaturization of the optical system.
Japanese Patent Laid-Open No. 2006-003913 discloses a DOE formed by bringing resin materials in contact with each other or stacking them with each other. Moreover, Japanese Patent Laid-Open No. 11-271514 discloses a DOE formed by bringing a glass material and a resin material in contact with each other.
Furthermore, Japanese Patent Laid-Open Nos. 2000-098118 and 2003-227913 disclose DOEs formed by using a glass material and a resin material. The DOE disclosed in Japanese Patent Laid-Open No. 2000-098118 is a two contacted layer DOE in which two diffractive gratings are disposed in contact with each other, and the DOE disclosed in Japanese Patent Laid-Open No. 2003-227913 is a stacked DOE in which plural diffractive gratings are stacked. These DOE appropriately select material forming the respective diffractive gratings and grating thicknesses of the respective diffractive gratings to achieve a high diffraction efficiency in a wide wavelength range for a specific order diffracted light. The diffraction efficiency is a ratio of a light intensity of the specific order diffracted light to that of entire light that is transmitted through the DOE.
The optical systems disclosed in Japanese Patent Laid-Open Nos. 2006-003913, 11-271514, 2000-098118 and 2003-227913, each of which includes the diffractive optical element, can have a sufficient chromatic aberration reduction effect. However, the grating thickness of the diffractive optical element in each of these optical systems is large, which may cause decrease of the diffraction efficiency when light obliquely enters to the diffractive optical element. The decrease of the diffraction efficiency increases unnecessary order diffracted light, which causes deterioration of imaging performance of the optical system due to flare or ghost and decrease of contrast of an image formed by the optical system.